docs: def url_from_rdf(rdf, prop):
      '''
      Try to extract An URL from a resource property.
      It can be expressed in many forms as a URIRef or a Literal
      '''
      value = rdf.value(prop)
      if isinstance(value, (URIRef, Literal)):
          return value.toPython()
      elif isinstance(value, RdfResource):
          return value.identifier.toPython()